Introduction to Git Basics

commit e1c7ae40152ee163f477863215e77917a863faa3

Author: Roy Lee <[email protected]>

Date: Sun Oct 14 12:10:30 2012 -0700

git: Introduction to GIT basics

Working tree, stage and database

Manipulating branches

Collaborating with remote repositories

Signed-off-by: Roy Lee <[email protected]>

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We have a small project,

and we want to manage it with GIT.

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empty... stage

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working tree

First, we create a git repository with „git init‟

and initializes the stage (a.k.a. index or cache)This creates a .git directory, which stores the whole git stuff,

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stageworking tree

Then we add the working tree to the stage.

This is called „staging‟.

staging

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Initial Commit

Finally, commit the stage to the database.

stageworking tree database

That‟s it! You now have a GIT repository

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$ git init # Initialize the git database (the .git directory)$ git add [ <paths> | <pattern> | . ] # Add files to the stage

# Commit the stage to the database with a specified log message# (without the “-m” options, git prompts the editor for asking log message)$ git commit -m “Initial commit”

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Now, we edit some files


HEAD

master

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Stage the changes.


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And commit the stage to the database.

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Another Commit


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Changes committed to or checked out from database

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Another Commit


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must get into the staging area

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two different things since the last commit.

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This time we have two files changed for

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we can do this incrementally by only staging


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Rather than committing them “all at once”,

“partial” of the changes.

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# Diff the working tree and the stage$ git diff

# Diff the stage and the HEAD commit$ git diff --staged

git diff git diff --staged

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Commit I

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Commit A

stageworking tree database$ git status# Changes to be committed:# (use "git reset HEAD <file>..." to unstage)## modified: T/Y## Changed but not updated:# (use "git add <file>..." to update what will be committed)# (use "git checkout -- <file>..." to discard changes in working directory)## modified: T/Z

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staged changes

unstaged changes

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Another Commit

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Commit the stage to the database.

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# Moving a tracked file around involves not only the working tree, but also the stage.$ mv T/Y Y$ git add Y$ git rm T/Y# ‘git mv’ simplify this with a single step.$ git mv T/Y Y

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$ git log # Browse the commit history$ git log --stat # Show diffstat$ git log -p # Show the logs in ‘patch format’$ git log -n2 # Show no more than two commits$ git log a..b # Show the commits between (a, b]

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# (1) Adjust the HEAD pointer (and the pointer pointed by it) to the <commit># (2) Reset the stage to <commit># (3) Checkout the files from the stage to working tree$ git reset --soft [<commit>] # (1), $ git reset --mixed [<commit>] # (1) + (2), this is the default behavior$ git reset --hard [<commit>] # (1) + (2) + (3)

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HEAD HEAD

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master

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# To restore the file contents from the cache to the working tree, use$ git checkout <paths> # (2)

# To retrieve the file contents from a specific commit, use$ git checkout <commit> -- <paths> # (1) + (2)

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branch A HEAD

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There are four types of object in the database.

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Tagv0.01

blob

tree

tagcommit

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Every object has an unique ID calculated by

taking the SHA1 hash of its contents.

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Tagv0.01

blob

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Objects are compressed and uniquely stored as

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blob

treeInitial

CommitAnother Commit

Tagv0.01

tagcommit

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Another Commit

Tagv0.01

blob

tree

tagcommit

delta

Similar contents could be stored as deltas.

loose objects or packfiles.

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Another Commit

A default branch master is created when you „clone‟ or „init‟ a git repo.

A branch can be seen as a chain of commits.

A branch name is simply a reference to the top most commit.

I A B C

HEAD

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Another Commit

# Creating a new branch on the current commit.$ git branch bak1

# Showing the commit history in a compact format $ git log --abbrev-commit --pretty=oneline HEAD25a0510 log messages for C7731a6d log messages for Bd2e8d8a log messages for Aa8272ea log messages for I

# Creating branches on the previous commits$ git branch d2e8d8a JK$ git branch 7731a6d XY

I A B C

XY

JK

HEAD

master

bak1

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Another Commit

# Active references are located in the .git/refs/ while inactive references might be packed in .git/packed-refs$ tree proj/.git/refs.git/refs/├─heads│ ├── JK│ ├──master│ └──XY└─ tags

└── v0.01$ cat proj/.git/refs/heads/JK3bf7886705b4ea7189f046fa5258fdf6edcdbe23$ cat proj/.git/HEAD # The HEAD usually (but not necessarily) points to another pointerref: refs/heads/master

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bak1

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Similarly, a ~N suffix refers to the previous N commit.

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HEAD^master~1XY^^XY~2

with a ^N symbol refers to its N-th parent commit.

XY^

A Branch name or a commit ID suffixed

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Another Commit

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jkJ

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The HEAD pointer refers to the current branch.

Switching between branches with ‟git checkout‟.

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# Switching between branches is also done by ‘git checkout’ without given the <paths># Recall that we used the ‘git checkout *tree-ish+ <paths>’ for checking out the content of a specified commit?

# This time the ‘git checkout’ command switches branches by updating the stage, working tree, and HEAD $ git checkout <tree-ish>

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HEADbranch A HEAD

master3

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# In cases where you have local changes, git compares your HEAD and TARGET commits for those modified files.# If the contents of the modified files on both commits# match : the local changes are preserved, and the checkout succeed# do not match : the checkout will be refused to avoid overwrite those local changes

(Thanks to Jeff King and Junio Hamano for their elaborations on how this works.)

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master

We‟d like to rebase JK and XY branches onto masterlike this

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J K JK HEAD

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master HEAD

Let‟s begin with rebasing XY onto master

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Find the common ancestor commit of both branches

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Checkout the tip commit of the target branch

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Apply the commits to the rebasing branch

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X Y XYX Y HEAD

Now, let‟s rebase JK onto master

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HEAD

Find the common ancestor commit of both branches

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HEAD

Checkout the tip commit of the target branch

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Apply the commits to the rebasing branch

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J K JK HEAD

Done!

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# The trace of the HEAD$ git reflogd5462a0 HEAD@{0}: rebase finished: returning to refs/heads/JKd5462a0 HEAD@{1}: rebase: log messages for Ka024870 HEAD@{2}: rebase: log messages for J6103ef9 HEAD@,3-: checkout: moving from JK to 6103…016564d HEAD@{4}: commit: log messages for K25a0510 HEAD@{5}: commit: log messages for J

# Browse the log$ git log --abbrev-commit --pretty=oneline HEAD@{4}016564d log messages for K25a0510 log messages for Jd2e8d8a log messages for Aa8272ea Initial

# Rescuing the commit with a “reference”$ git branch HEAD@{4} old_JK

old_JK

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0 & 1

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Apply the commit C to the JK branch with ‘git cherry-pick’

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Revert the commit A with ‘git revert’

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# Suppose we have a proj host on my-server.com

alice server bob

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# Alice clones it to her local machine.alice $ git clone [email protected]:/var/git/proj.git

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# And Bob also clones it on his machine.bob $ git clone [email protected]:/var/git/proj.git

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# Alice commits to her local branch.alice $ git commit -a -m “message for J”alice $ git commit -a -m “message for K”

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# Bob also commits to his local branch.bob $ git commit -a -m “message for B”

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# Bob pushes his commit to the server with either one of the following commands.bob $ git push [email protected]:/var/git/proj.git HEAD:masterbob $ git push origin HEAD:masterbob $ git push

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# Then pushes her local commits to the server with either one of the following commands.alice $ git push [email protected]:/var/git/proj.git HEAD:masteralice $ git push origin HEAD:masteralice $ git push

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# But the push fails with the following messageTo [email protected]:/var/git/proj.git ! [rejected] master -> master (non-fast-forward)error: failed to push some refs to ‘[email protected]:/var/git/proj.git'To prevent you from losing history, non-fast-forward updates were rejectedMerge the remote changes before pushing again. See the 'Note aboutfast-forwards' section of 'git push --help' for details.

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# Alice updates her local repository with either one of the following commandsalice $ git fetch [email protected]:/var/git/proj.gitalice $ git fetch origin

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# Alice rebases his branch onto the origin/masteralice $ git rebase origin/master masterFirst, rewinding head to replay your work on top of it...Applying: message for JApplying: message for K

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# Alice pushes his commits to the server.alice $ git remote add bob alice@bob-pc:/var/git/public/proj.gitalice $ git fetch bob

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origin/HEAD # Bob commits another changes to his local branchbob $ git commit -a -m “messages for X”

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origin/HEAD # Alice adds Bob’s repository as another remote repository.alice $ git remote add bob alice@bob-pc:/var/git/public/proj.gitalice $ git fetch bob

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# List remote repositoriesalice $ git remote -vorigin [email protected]:/var/git/public/proj.git (fetch)origin [email protected]:/var/git/public/proj.git (push)bob alice@bob-pc:/var/git/public/proj.git (fetch)bob alice@bob-pc:/var/git/public/proj.git (push)

# The references can be found at .git/refs/alice $ tree proj/.git/refsproj/.git/refs/├─heads│ └──master├─ remotes│ ├─bob│ │ ├──HEAD│ │ └──master│ └─origin│ ├──HEAD│ └──master └─ tags

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$ git init$ git add$ git rm$ git mv$ git commit$ git reset$ git clean$ git stash

$ git tag$ git diff$ git status$ git log$ git show$ git grep

$ git branch$ git checkout$ git cherry-pick$ git revert$ git rebase$ git show-branch

$ git clone$ git fetch$ git push$ git pull$ git remote

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# Then pushes her local commits to the server with either one of the following commands.# ~/.gitconfig

[user]name = Your Nameemail = Your E-mail Address

[color]branch = autodiff = autostatus = auto

[alias]timeline = log --oneline --dec --branchesl = log --decbr = branchst = status -sstu = status -s -unoco = checkout

[core]filemode = false

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# Try out the following commands, and add it to your ~/.bash_profile or similar

# Source the bash completion script$ source /etc/base_completion.d/git

# git 1.7.X+ has split the prompt and shell completion to separate files$ source /etc/base_completion.d/git$ source /etc/base_completion.d/git-prompt.sh

# After the right scripts are sourced, __git_ps1 is available shows the current status/branch.~/git_repo/some_proj $ __git_ps1(wrk)

# You can incorporate __git_ps1 to your PS1 environment variableexport PS1='\[\e[1;32m\]\u@\[\e[1;33m\]\h\[\e[1;34m\] \w$(__git_ps1 "\[\e[1;36m\] <%s>\[\e[1;34m\]") $\[\e[m\] '

Git configurations

git config, .git/config, ~/gitconfig

Rebasing commits interactively

git rebase -i HEAD~n

Manipulate working tree and the stage interactively

git add -I

git add -p

git checkout -p

Apply patches

git am, git apply

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Technology

Introduction to Git Basics